Can single celled organisms be conscious?
Margulis and Sagan (1995) described consciousness as basically ‘awareness of the outside world’. Any organism that exhibits sensory perception and responds to it has some awareness of its outside world (Trewavas and Baluska, 2011). But awareness of a world outside of itself also requires awareness of itself, i.e., self-recognition. C. elegans is aware of the outside world and uses it to profit from experience but so does Physarum polycephalum, a single but multinucleate cell. This coenocyte learns to anticipate simple repetitive signals, such as three electrical or touch cues, by momentary reductions in growth rate. It responds to the expected but un-provided fourth signal by equivalent growth rate reductions (Saigusa et al., 2008). There are many similar capabilities expressed by other single cells (Trewavas, 2014). Physarum recognizes itself because when any of its growth lobes meet, growth usually ceases; it will not try to exploit different branches of itself. Any network as complex as the interactome and connectome, will be conscious and if we ever build an artificial network with this kind of structure, it will likely be conscious too.
An alternative more limited definition of consciousness is the use of mental images to regulate behavior. Because animals have always had to move to find food, predation of animal on animal evolution refined sensory and motor equipment and joined the two with a rapid connection and later assessment system of nerve cells compacted into a brain. The ancestors of plants having acquired photosynthesis found, beneficially, light energy ubiquitously distributed but to contain the osmotically active products required a relatively rigid wall enormously inhibiting individual movement. The individual plant containing many millions of cells is a self-organizing, complex system with distributed control permitting local environmental exploitation but in the context of the whole plant system. Consciousness is thus not localized but is shared throughout the plant in contrast to the more centralized location in the animal brain. But from a system framework, the real plant is the individual together with its environmental connections and its self-constructing niche (Trewavas, 2014).